DESCRIPTION: This is a renewal of a long-standing study of growth regulation in estrogen target cells. The grant would extend ongoing studies to determine how serum proteins inhibit growth of mammary cells in culture. Three specific aims are presented which would identify the specific regions of serum albumin which block growth of estrogen sensitive cells, characterize the active fraction of serum from albumin negative patients, and finally, assess the role of a serum albumin receptor in the mammary cells which might explain the effects of the growth repressor in serum.